Several processes are known for manufacturing window shades of folded material. Two basic varieties of these shades of relevance here are as follows. A first, pleated type consists of a single panel of corrugated material. The other is a more complex cellular type, where stacked folded strips form a series of collapsible cells. This latter type is known to have favorable thermal insulation properties, because of the static air mass which is trapped between the layers of material when the cells are in the expanded position. The single-panel type, on the other hand, is favored for its appearance in some cases, and is less expensive to manufacture.
There is considerable difference in the method of manufacture of the single-panel and cellular shades. The former has heretofore been made by repeatedly folding the material across its width, so that it becomes pleated. Among the difficulties with this approach is the need to repeatedly make narrow parallel folds transversely across a wide sheet of material of continuous length. Unless highly exacting conditions are maintained, the folding process can fall out of alignment. Also, substitution of materials is cumbersome because sheets of material and not strips are involved. Further, single-panel shades are relatively weak, structurally, as compared to cellular shades. Retention of pleat shape is a significant problem with most single-panel shades, and is particularly severe where non-woven or sheer fabrics are used. Another disadvantage is the necessity of using multiple sheets joined at seams where large shades are desired.
There are several methods for producing the cellular shades. Most similar to the pleated, single-panel method is the Anderson U.S. Pat No. 4,685,986. This joins together two single-panel pleated lengths of material by adhesively bonding them together at opposing pleats. The adhesive bonding step limits the problem of pleat retention noted above with respect to pleated shades. Other methods depart from the Anderson Patent by joining together series of longitudinally folded strips, rather than continuous sheets of pleated material. Such methods are shown in Colson U.S. Pat. No. 4,450,027, and in Anderson U.S. Pat. No. 4,676,855. In the Colson Patent strips longitudinally folded into a U-shape are adhered on top of one another, whereas in the Anderson Patent these strips are Z-shaped and are adhered in an interlocking position.
The strip joining method shown in the Colson patent has a number of desirable attributes, while providing a highly desirable thermally insulative shade. First, the alignment problems inherent in folding large sheets of material transversely to make pleated shades are largely avoided. Second, substitution of materials is easier because strips and not sheets of material are involved. Third, structural strength is increased, which gives greater pleat retention and allows for more lightweight materials to be used. Fourth, larger shades can be made without the need for seams. Finally, the speed of production of such cellular shades is at least as fast as that of single-panel pleated shades made by usual methods.